The invention relates to musical instrument audio power amplifiers for driving loud speakers. In particular, the invention relates to audio amplifiers for guitars and other musical instruments having frequency selective damping factor controls for improving the sound emitted by loud speakers over a full range of audio inputs and particularly at low frequencies near system resonance.
The damping factor of a power amplifier is sensitive to load impedance. Although loud speakers usually have a nominal impedance, the actual impedance varies considerably over its range of operating frequencies. In particular, the impedance of a loud speaker increases with increasing frequency due to the inductance of the loud speaker coil. At low frequencies the impedance of a loud speaker increases generally to a maximum at free air resonance which is the function of the mechanical characteristics of the speaker and its enclosure. When mounted in an enclosure the impedance peaks at the so called system resonance which is a function of the speaker and enclosure characteristics. It is common practice to select speakers and enclosures to obtain a desired sound. It is not entirely clear what effect damping factor has on the sound quality of a loud speaker, because that is subjective. However, there is general agreement that a change in the damping factor can significantly affect the volume of loud speaker sound.
Generally, the damping factor of a power amplifier is defined as the ratio of the load impedance to the output impedance of the amplifier. ##EQU1## where Z.sub.L is the load impedance and Z.sub.O is the amplifier output impedance.
It is also generally accepted that the damping factor may be defined in terms of full load voltage and no load voltage as follows: ##EQU2## where V.sub.rms (FL) is the amplifier output in rms at full load and V.sub.rms (NL) is the amplifier output voltage in rms at no load or open circuit.
When damping factor is defined in terms of the impedance, an amplifier with a high damping factor is viewed as having a low output impedance. Such an amplifier has particular use in high fidelity applications in which the speaker produces a generally flat frequency response. This is generally referred to as a so called "tight" or "controlled" sound because the speaker cone has controlled or limited motion.
An amplifier with a low damping factor is viewed as having a high output impedance. Such an amplifier has less control over the loud speaker and thus the cone motion is not as controlled. For most guitar or instrument applications, amplifiers having relatively low damping factors are desirable because they are believed to make the guitar sound better to the musician and audience alike. The low damping factor improves both the high and low frequency response and causes the associated enclosure to produce more low end output at or near the enclosure resonance. The sound produced by speakers driven by a low damping factor amplifier are said to "flop" or "overshoot" and the low end sound is "boomy". In any event, sound quality is subjective to the listener. All that can be said is that a boomy sound is commercially desirable for guitar amplifier applications.
When the damping factor is examined in terms of its voltage relationships it is thought that a lower damping factor can advantageously affect the high and low frequency response of a loud speaker by increasing the power delivered to the speaker as the impedance increases. For example, using the voltage relationship referred to above, an amplifier having a damping factor of one (1) and producing a full load output voltage of 20 volts at the mid band frequencies can produce 40 volts at no load. In other words, if the output of the amplifier is open circuit, the output is 40 volts. Similarly, it can be shown that an amplifier with a damping factor of 100 and delivering 20 volts at full load produces about 20.2 volts at no load. Thus, a high damping factor amplifier has a relatively constant output voltage as the impedance of the speaker increases. Unfortunately, as explained below, the power delivered on the speaker from an amplifier with a high damping factor is reduced at both high and low frequencies which results in a weak or poor sound for guitar applications.
Power is a function of the output voltage and may be defined as: P=V.sub.rms.sup.2 /Z.sub.L. As the load impedance increases the actual power delivered to the load decreases. Further, as the output voltage increases the power delivered to the load increases by the square of the voltage. In the above example, if the damping factor is high there is a significant reduction in output power because the output voltage does not increase in proportion to the increase in speaker impedance. If the damping factor is low, the power delivered to the load is not reduced as much because the output voltage increases to a greater extent with increasing speaker impedance, and the speaker sound is thereby enhanced.
Damping factors less than 1 for example, 0.25 or less are not uncommon. However, simple and effective control of the damping factor values particularly over the low frequency range has not been achieved. There are controls, sometimes referred to as presence controls, which provide boost to the amplifier output at high frequencies and add so called "brilliance" or "edge" to the amplifier which cannot be duplicated with conventional high end equalization or boost or treble circuitry.
Presence control in the form of a potentiometer (pot) and parallel capacitor coupled to the wiper of the pot in the cathode circuit of a tube amplifier is known. The presence control improves high end performance by lowering the damping factor. There is also a known so called global damping control for tube amplifiers which inserts a high resistance in series with the feedback resistor from the load. In such an arrangement the increased impedance in the feedback circuit decreases the damping factor of the amplifier without regard to frequency. Although it is desired to enhance the low frequency or "bottom" using this control, functionally the global damping control does not achieve satisfactory results for two reasons. First, the range of the control has been limited. Second, the damping control adversely affects the high end settings. Presence controls and variable global damping have not been used in solid state amplifiers because they interfere with the necessary feed back circuits.
Tube type amplifiers normally have a characteristic low damping factor resulting from an inherently low feedback requirements. Thus, further reduction of feedback to thereby reduce the damping factor is achieved by reducing the elementary feedback. However, tubes as amplifier components although effective and in many cases preferable, are being used less. This is so mainly because solid state devices have virtually supplanted most applications for tubes and thus there has been a general reduction in demand for tubes and tube manufacturing capability world wide. In effect tubes are obsolete for many applications and are becoming difficult to obtain for remaining applications where they are thought to be superior.
Solid state amplifiers have a characteristically high damping factor resulting from required high feed back requirements. This characteristic makes it difficult to reduce the damping factor without adversely affecting the feedback circuitry. However, because of the reduced availability of tubes, a low damping factor solid state amplifier is needed.